Ambiguous Optical Illusions

Ambiguous figures are 'two pictures in one' - looked at one way, you
see one thing (e.g. a vase), but looked at another, you see something
else (e.g. two faces in profile).

They are somewhat similar to unstable figures, in that there are two
ways to interpret each one, but unstable figures don't contain
different subjects; they contain one subject that flips perspective.

A very famous illusion - young lady or old hag?

In 1915, a cartoonist named W.E. Hill first published this drawing.
It's hard to see what it's supposed to be.
Is it a drawing of a pretty young girl looking away from us?
Or is it an older woman looking down at the floor?
Well, it's both. The key is perception and what you expect to see.
Here's a hint: The young girl's necklace is the older woman's mouth.
The young girl's chin and jaw are the older woman's nose.

Is it a man playing a saxophone, or a girl's face?

A wine goblet - or two faces in profile?

The white triangle looks very real, although there's no triangle per se.